Which one of the following statements is not correct?

Option (4) is not correct. Barium and strontium are not typically found in the free (native) state but occur in minerals—e.g., celestite and other strontium/barium ores—rather than as native metals [4]. The other statements are correct: Plaster of Paris (CaSO4·½H2O) hydrates back to gypsum on mixing with water, and cations such as Na+ accelerate gypsum setting, so NaCl can increase the setting rate [3]. Gypsum is widely used in the cement industry and is added to Portland cement to control/retard the setting of clinker, thus slowing the rate of setting [2]. Alkaline‑earth metals commonly form hydrated salts (e.g., calcium sulfate dihydrate gypsum) as part of their chemistry [5].

Sources

1. [4] https://pubs.usgs.gov/bul/0540t/report.pdf
2. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Plaster of Paris > p. 33
3. [3] https://www.govinfo.gov/content/pkg/GOVPUB-C13-5cd6244272fbe7414c372b367c0c4da7/pdf/GOVPUB-C13-5cd6244272fbe7414c372b367c0c4da7.pdf
4. [2] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 7: Resources > Natural Resources of India > p. 28
5. [5] https://www.sciencedirect.com/topics/engineering/nuclear-fuel-system